German Patent Publication (DE-OS) 2,805,370 discloses a method and an aluminized coating for drilled passages in turbine blades. The known aluminized coating has the disadvantage that it is deposited at low temperatures between 700.degree. C. and 850.degree. C., whereby an aluminum diffusion into the surface of the component is prevented. For aluminizing components, the known method passes a carrier gas, such as hydrogen, through aluminum trihalide, which at temperatures above 900.degree. C., is subsequently converted into aluminum subhalide over a pool of liquid aluminum or a liquid aluminum alloy. Thereafter, pure aluminum is deposited in inner bores of the structural component.
It is an essential disadvantage of the known method that for converting thermally stable aluminum trihalide into aluminum subhalide, liquid aluminum or aluminum alloys must be formed within the deposition reactor. The resulting aluminum mono-halide formed in the process is impure. Rather, a substantial proportion of at least 20% aluminum trihalide remains in the mixture, whereby the aluminum deposition rate is reduced. A further disadvantage of this method is seen in that it requires the installation of melting crucibles in the deposition reactor.
In addition, the absorption and formation of aluminum monohalide is limited by the limited reaction surface area of the melt in the crucible.
French Patent Publication FR-PS 1,433,497 discloses an aluminum gas phase deposition process, wherein aluminum or aluminum alloy particles are used as an aluminum source and the source temperature is too low for the aluminum source to melt. A halogen gas is passed through the aluminum source for forming aluminum halides. The disadvantage of this known method is its low aluminum source temperature, which prevents achieving high deposition rates.
U.S. Pat. No. 4,132,816 discloses how to achieve higher deposition rates by adding activators, such as alkaline or alkaline earth halides or complex aluminum salts to the aluminum source. These additives, however, disadvantageously reduce the purity of the aluminized coating, especially since the substances admixed to the source material comprise not only activators, but also oxides, such as aluminum oxide.